Question: Which of these numbers is prime? ${64,\ 65,\ 69,\ 91,\ 97}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 64 are 1, 2, 4, 8, 16, 32, and 64. The factors of 65 are 1, 5, 13, and 65. The factors of 69 are 1, 3, 23, and 69. The factors of 91 are 1, 7, 13, and 91. The factors of 97 are 1 and 97. Thus, 97 is a prime number.